What was the most dangerous programming mistake you have made in C?

Question

I am an intermediate C programmer. If you have made any coding mistake that you came to know later that it was the most hazardous / harmful to the total application please share

Answer 1

When a pointer is first allocated, it does not have a pointee.

The pointer is "uninitialized"

A dereference operation on a bad pointer is a serious runtime error.

If you are lucky, the dereference operation will crash or halt immediately (Java behaves this way).

If you are unlucky, the bad pointer dereference will corrupt a random area of memory, slightly altering the operation of the program so that it goes wrong some indefinite time later. Each pointer must be assigned a pointee before it can support dereference operations.